Printed circuit boards (PCBs) are widely used to hold and electrically connect selected electronic components. Printed circuit boards are comprised of copper electrical interconnects separated by levels of insulating dielectric materials. In the horizontal xy plane, the copper interconnect lines are patterned on each of the individual layers and the layers joined to form a composite. Through-holes are then drilled in the vertical z direction from top to bottom of the composite, passing through each layer of the composite as well as through the copper landing pads on the layers to which electrical connection is required. A seed layer followed by one or more copper layers is then applied to the interior surface of the drilled through-holes.
A conventional method of manufacturing PCBs is depicted in FIG. 1, and consists of the following steps: 1) sub-laminates of epoxy pre-impregnated glass cloth (prepreg) are designed and prepared with copper interconnect lines on both sides of the prepreg, and the sub-laminates compiled with as many layers of signal, power and ground as are needed and joined together at elevated temperature and pressure, FIG. 1(a); 2) through-holes 10 are mechanically drilled through the PCB 12, FIG. 1(b) (internal surface features of through-hole not shown); 3) a seed layer 14 is applied to the surface of the PCB and the interior surfaces of the through-holes, FIG. 1(c); 4) a thin “plating base” of electroless copper 16 is applied to the seed layer 14, FIG. 1(d); 5) photoresist 18 is applied and patterned on the external surfaces of the PCB, FIG. 1(e); 6) copper 20 is electroplated using the plating base as an electrical common layer, FIG. 1(f); and 7) the photoresist 18 is stripped and the exposed plating base is removed by etching, FIG. 1(g).
Following the stripping of the photoresist, the thin copper layer that had been protected by the photoresist is etched away to form a copper interconnect pattern on the external surfaces of the PCB, and leaving plated through-holes with a conducting copper layer on most, if not all, of the interior surface of the through-holes. Thus, regardless of which two copper interconnect layers in the PCB are associated with the through-hole, the copper is present throughout the hole, i.e. from top to bottom of the PCB as well as extra copper 30 (copper stubs) at the through-holes, which are known to have detrimental effects on signal integrity.
The PCB is designed to connect any one copper interconnect line for any prepreg layer to another copper interconnect line of another prepreg layer via a plated through-hole. However, because the plated through-holes are formed after the multilayer structure is assembled, there are extraneous copper stubs 30 at the top and/or the bottom of the plated through-hole as explained above. These extraneous copper stubs 30 limit the electrical performance of the copper interconnection especially at high frequency, and therefore, they often need to be removed. A typical process for removing the copper stubs involves redrilling or “backdrilling” the plated through-hole to remove the extraneous and undesired copper to provide a backdrilled PCB as shown in FIG. 2.
The backdrilling process is time consuming, expensive and subjects the PCB to unnecessary damage, which affects the production yield. First, the process is time consuming because each through-hole must be carefully drilled only to a limited depth which varies from hole-to-hole. Second, the back drilling leaves a portion 32 of the stub 30 as part of the allowed tolerance related to drill depth. Third, some of the PCBs are damaged by the backdrilling and have to be discarded.